Continuously variable dynamic brake for a locomotive

ABSTRACT

This disclosure is directed to a traction motor drive system. The traction motor drive system may include a field winding subsystem comprising a field winding associated with a traction motor. The traction motor drive system may also include an armature subsystem arranged in parallel with the field winding subsystem. The armature subsystem may include an armature having first and second armature terminals and a grid resistor selectively electrically coupled in series with the armature. The armature subsystem may also include an armature chopper arranged in parallel with the grid resistor and electrically coupled in series with the armature. The armature chopper may be configured, when the grid resistor is electrically coupled to the armature, to conditionally conduct current to the armature.

TECHNICAL FIELD

The present disclosure relates generally to traction motor drivesystems, and more particularly, to systems and methods for dynamicbraking on a locomotive.

BACKGROUND

During dynamic braking, traction motors may function as generators toslow the movement of the locomotive by converting the kinetic energy ofthe locomotive into electrical energy. In rheostatic dynamic braking,grid resistors can be incorporated to dissipate the generated energy asheat. As the locomotive slows, the current generation of the armaturedecreases. To allow dynamic braking to continue as current generation ofthe armature continues to decrease, the field current is increasedtowards the maximum rated current of the field windings. Once themaximum rating of the field current is reached, the equivalentresistance of the grid resistor must be decreased for dynamic braking tocontinue to function. Lowering the equivalent resistance of the gridresistors permits dynamic braking despite lower rotational velocity ofthe motors. Early dynamic braking systems begin shorting out portions ofthe grid resistor to lower its equivalent resistance. This would extendthe range of locomotive speed over which dynamic braking was operable.The early dynamic braking systems present two problems.

First, these dynamic braking systems could not operate at slowlocomotive speeds. Early braking systems could not shunt out the entiregrid resistor without disconnecting the grid blower, a necessarycomponent for dynamic braking, which is in the same circuit as the gridresistor. Typically, once the locomotive has reached speeds of 6 mph orslower, the locomotive must rely solely on its mechanical brakingcomponents, like pneumatic brakes, until the locomotive has completelystopped. The increased use of friction-based braking systems producesunnecessary wear on these parts and requires more frequent repairs ofthe braking system. Thus, to increase the working life of thelocomotive's mechanical braking components, an extended range dynamicbraking system may be required.

Second, early rheostatic braking systems are only capable of extendedrange dynamic braking, and cannot produce a continuously linear brakingforce. Extended-range dynamic braking shorts out portions of the gridresistor at a series of discrete points, such that lowering theequivalent resistance of the grid resistor is accomplished in a seriesof steps. In a legacy system, each step down causes locomotive handlingproblems for the operator. During a step-down of the grid resistor, thelocomotive often lurches or lunges. The operator must be able to handleeach of these difficulties to safely control the locomotive. A brakingsystem capable of applying a continuously linear braking force is ableto gradually decrease the equivalent resistance of the grid resistor,eliminating the locomotive handling problems associated with step-downdynamic braking. Thus, to increase locomotive safety, a continuouslyvariable dynamic braking system may be required.

One solution for maintaining dynamic braking at low speeds is describedin U.S. Patent Application Publication No. 2009/0295315 A1 (“the '315publication”). The '315 publication is directed to a system thatpurportedly incorporates a dynamic braking system for a hybridlocomotive that works at low speeds.

The dynamic braking solution provided by the '315 publication is limitedto traction motors in which the armature and the field winding of thetraction motor are connected in series. This circuit requires additionalcontrol methods to overcome the instability resulting from the armatureand field windings being connected in series. For example, theseinstabilities can create a positive feedback condition across the fieldcoils, which will cause a runaway current build-up if not properlycontrolled. To implement rheostatic braking, the circuit of the '315publication requires the current path through the DC bus to be completedby a grid resistor. In this system, connecting the grid resistordirectly to the DC bus requires additional controls to prevent the gridresistor from unnecessarily dissipating power during normal motoring.Furthermore, the system cannot isolate the grid resistor in the event itmalfunctions. While the '315 publication purportedly allows for dynamicbraking at low speeds, the design makes the traction motor drive systemunnecessarily vulnerable to current instability and grid resistorbreakage.

The presently disclosed traction motor drive system is directed toovercoming one or more of the problems set forth above and/or otherproblems in the art.

SUMMARY OF THE INVENTION

In accordance with one aspect, the present disclosure is directed to atraction motor drive system. The traction motor drive system may includea field winding subsystem comprising a field winding associated with atraction motor. The traction motor drive system may also include anarmature subsystem arranged in parallel with the field windingsubsystem. The armature subsystem may include an armature having firstand second armature terminals and a grid resistor selectivelyelectrically coupled in series with the armature. The armature subsystemmay also include an armature chopper arranged in parallel with the gridresistor and electrically coupled in series with the armature. Thearmature chopper may be configured, when the grid resistor iselectrically coupled to the armature, to conditionally conduct currentto the armature.

In accordance with another aspect, the present disclosure is directed toa method of controlling an armature chopper arranged in series with anarmature and in parallel with a grid resistor to decrease the effectiveresistance of the grid resistor when the armature is generating current.The method may include monitoring a field current associated with afield winding, the field winding arranged in parallel with the gridresistor and the armature. The method may also include comparing thefield current with a threshold value. When the field current exceeds thethreshold value, the method may include calculating a duty cycleassociated with the armature chopper and switching the armature chopperat the calculated duty cycle to selectively bypass the grid resistorduring the portion of a period the armature chopper is on.

According to another aspect, the present disclosure is directed to alocomotive. The locomotive may include a plurality of axles and aplurality of pairs of wheels, each pair of wheels attached to one of theaxles. The locomotive may also include a plurality of armatures arrangedin parallel with one another, each having first and second armatureterminals. Each armature may be rotatably coupled to one of the axles.The locomotive may also include a plurality of field windings arrangedin series with one another, each field winding associated with arespective one of the armatures. The plurality of field windings may bearranged in parallel with the plurality of armatures. The locomotive mayinclude a plurality of armature choppers. Each armature chopper may beelectrically coupled to the first armature terminal of a respective oneof the armatures. The locomotive may include a plurality of gridresistors. Each grid resistor may be arranged in parallel with arespective one of the armature choppers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary locomotive that comprises a tractionmotor;

FIG. 2 provides a schematic of an exemplary traction motor drive system;

FIG. 3 provides a flowchart depicting an exemplary method of controllingan armature chopper; and

FIG. 4 provides a schematic of an exemplary traction motor drive systemincluding circuitry capable of isolating malfunctioning components.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary locomotive 100 in which systems andmethods for dynamic braking may be implemented consistent with thedisclosed embodiments. Locomotive 100 may be any electrically poweredrail vehicle employing DC traction motors for propulsion. Furthermore,any electrically powered vehicle employing DC traction motors forpropulsion could also incorporate the systems and methods for tractionmotor isolation consistent with the disclosed embodiments. According tothe exemplary embodiment illustrated in FIG. 1, locomotive 100 mayinclude six pairs of wheels 101, with each pair of wheels 101 attachedto an axle 102 that is rotatably coupled to a traction motor 103.Traction motors 103 may each include an armature 104 and a field winding105. As locomotive 100 uses a DC traction motor system, traction motor103 may comprise separate circuits for armature 104 and field winding105. FIG. 2 illustrates the relationship between armature 104 and fieldwinding 105 within a traction motor drive system 200.

Traction motor drive system 200 includes a plurality of mechanical andelectrical components that cooperate to propel locomotive 100. Tractionmotor drive system 200 may be divided into two distinct but cooperativesubsystems, a plurality of armature subsystems 201 and a field windingsubsystem 202. Armature subsystems 201 and field winding subsystem 202are each connected to a positive voltage source 203 and a negativevoltage source 204. As shown in FIG. 2, traction motor drive system 200comprises a single field winding subsystem 202, which includes fieldwindings 105 for each traction motor 103 of traction motor drive system200. Traction motor drive system 200 comprises a separate armaturesubsystem 201 for each armature 104. In traction motor drive systemsthat comprise multiple traction motors 103, armature subsystems 201 arearranged in parallel with each other.

Field winding subsystem 202 may be connected in parallel with theplurality of armature subsystems 201. Within field winding subsystem202, each field winding 105 may be connected in series with one another.In the exemplary embodiment, there are six armature subsystems 201, eachcorresponding to one of six traction motors 103. For clarity, FIGS. 2and 4 show only three of the six armature subsystems 201. Threeadditional armature subsystems 201 may be connected in parallel toarmature subsystems 201 shown in FIGS. 2 and 4. Of course, thisembodiment may be altered to accommodate a different number of tractionmotors 103 by changing the number of armature subsystems 201 and thenumber of field windings 105 within field winding subsystem 202.

In addition to field windings 105, field winding subsystem 202 may alsoinclude components necessary to operate field windings 105 during normaloperation. For example, field winding subsystem 202 may also include apair of field polarity switches 205, 206, a reverser 207, and a fieldchopper 208. Field chopper 208 may be arranged in series with theplurality of field windings 105.

Field chopper 208 may embody a power-regulation device configured toregulate current through field windings 105. By controlling the currentthrough field windings 105, field chopper 208 may be configured toregulate the torque of traction motors 103. By way of example, whenlocomotive 100 begins to pull a load, it is the nature of tractionmotors 103 to require high amounts of current at low generator voltageto provide the torque needed to initially move locomotive 100 and itsload. As locomotive 100 accelerates, the requirement for current reduceswhile the applied voltage increases. Field chopper 208 responds to thisdemand.

By manipulating the direction of current flow through field windings 105using field polarity switches 205, 206 and reverser 207, traction motordrive system 200 can control the direction of rotation of tractionmotors 103, allowing locomotive 100 to travel in both the forward andreverse directions. This ability to change the direction of current flowthrough field windings 105 may also be used during dynamic braking.

Reverser 207 is configured to act as a connection point to the series offield windings 105. Field polarity switches 205, 206 are configured toswitch between the different connection points of reverser 207. Firstfield polarity switch 205 may be connected to field chopper 208 andsecond field polarity switch 206 may be connected to negative voltagesource 204. Field polarity switches 205, 206 may be configured to changethe polarity of field windings 105.

Reverser 207 may connect to the series of field windings 105. Reverser207 has four leads. The first pair of leads connects directly to theseries of field windings 105. The second pair of leads is a set ofconnection points that field polarity switches 205, 206 can engage. Whenthe directions of field polarity switches 205, 206 are switched, theswitches connect to different leads of reverser 207, which effectivelyreconfigures field winding subsystem 202, reversing the direction ofcurrent flow through field winding subsystem 202 and its field windings105.

Field polarity switches 205, 206 of the exemplary embodiment may besingle-pole, double-throw switches, although field polarity switches205, 206 may comprise any combination of components capable of switchingthe direction current flow through field windings 105. In a firstposition, field polarity switches 205, 206 connect directly to the firstpair of connection points of reverser 207. In this mode, field polarityswitches 205, 206 allow current to flow directly through the series offield windings 105. In a second position, field polarity switches 205,206 connect to the second pair of connection points of reverser 207. Inthis mode, the current must flow through reverser 207 before flowingthrough field windings 105 in the opposite direction that it flows whenfield polarity switches 205, 206 are in the first position.

As shown in FIG. 2, each armature subsystem 201 may include armature104, a motor-brake switch 209, and other components for facilitatingdynamic braking. Armature subsystem 201 may include the components tooperate armature 104 during both powering mode and braking mode. Each ofthe plurality of armatures 104 comprises a first armature terminal and asecond armature terminal. The first armature terminal may be coupled topositive voltage source 203. The second armature terminal is coupled tomotor-brake switch 209, which may switch between powering mode andbraking mode. The first terminal of armature 104 may connect to positivevoltage source 203 through the dynamic braking components.

Motor-brake switch 209 can switch the configuration of armature 104 tochange between powering mode and braking mode. In one embodiment,motor-brake switch 209 may be a single-pole, double-throw switchconnected to the second terminal of armature 104. During normal poweringmode, motor-brake switch 209 may connect the second terminal of armature104 to negative voltage source 204. During braking mode, motor-brakeswitch may connect the second terminal of armature 104 to positivevoltage source 203.

A brake contactor 210 may cooperate with motor-brake switch 209 toswitch traction motor drive system 200 from powering to braking. Brakecontactor 210 may electrically connect a grid resistor 211 to tractionmotor drive system 200. Traction motor drive system 200 may include aplurality of brake contactors 210, each connected between a respectivearmature 104 and a respective grid resistor 211. During powering mode,brake contactor 210 remains open, electrically isolating grid resistor211 from armature 104. During braking mode, brake contactor 210 closes,providing an electrical connection between armature 104 and gridresistor 211 to allow grid resistor 211 to dissipate the excess powerproduced during dynamic braking. Brake contactor 210 may be any switchor contactor capable of performing this function. In one exemplaryembodiment, brake contactor 210 is a single-pole, single-throw switch.Brake contactor 210 may be controlled manually by an operator command,or it can change automatically when motor-brake switch 209 is moved intoa braking mode.

When motor-brake switch 209 and brake contactor 210 are switched intobraking mode, traction motor drive system 200 may be configured act as a“dynamic brake” for locomotive 100. Dynamic braking is anelectrically-assisted braking method that slows locomotive 100 duringnormal road operation and when descending steep mountain grades. Duringdynamic braking, the position of motor-brake switch 209 configurestraction motors 103 to act as generators. The momentum of locomotive 100provides the mechanical energy for rotating armatures 104 relative tofield windings 105, thereby converting the kinetic energy associatedwith the momentum of locomotive 100 into electrical energy. As will beexplained in greater detail below, the electrical energy can be quicklydissipated by grid resistors 211. The resistance provided by gridresistors 211 increases the mechanical drag on armature 104, causingwheels 101 of locomotive 100 (which are connected to armature 104) toturn more slowly.

As explained, traction motor drive system 200 may include components fordissipating this generated electrical energy. These components mayinclude grid resistors 211 and an armature chopper 212. Grid resistors211 dissipate generated power during rheostatic dynamic braking as heat.Grid resistors 211 may be fixed resistors. In one embodiment, theresistance of grid resistors 211 may be around 0.632 Ohms As shown inFIG. 2, traction motor drive system 200 may include a plurality of gridresistors 211, each arranged in series with a respective one of thearmatures 104. Grid resistor 211 may be arranged in parallel with anarmature chopper 212.

Armature chopper 212 is electrically connected to the first armatureterminal of an associated armature 104. In normal powering mode,armature chopper 212 regulates the current through armature 104 relativeto the current through field windings 105. During dynamic braking,armature chopper 212 can be selectively operated to regulate theequivalent resistance of the grid resistor 211 to provide a continuouslyvariable resistance value for increasing the effective speed range fordynamic braking.

Armature chopper 212 may be any switched DC current regulation device.For example, armature chopper 212 may comprise a DC-DC chopper capableof handling the power requirements of traction motor drive system 200.In one embodiment, armature chopper 212 may use one or more gateturn-off (“GTO”) thyristors. Alternatively, armature chopper 212 may usetransistors to control the current flow through grid resistors 211.

By changing its duty cycle, armature chopper 212 can vary the equivalentresistance of grid resistor 211. When armature chopper 212 is “on,” itis conducting, so that current is shunted around grid resistor 211.Alternatively, when armature chopper 212 is “off,” it operates as anopen circuit and current may flow through grid resistor 211. By way ofexample, when armature chopper 212 operates at 100% duty cycle duringdynamic braking, the effective resistance of grid resistor 211 would benegligible. Alternatively, if armature chopper 212 operates at 0% dutycycle during dynamic braking, the effective resistance of grid resistor211 would be the actual resistance of grid resistor 211. Armaturechopper 212, therefore, may vary the effective resistance of gridresistor 211 by changing its duty cycle between 0% and 100%, dependingupon a desired effective resistance. By providing an electricallycontrollable mechanism, armature chopper 212 can be used toelectronically decrease the effective resistance of grid resistor 211,allowing a more gradual decrease of braking force produced by armatures104 as locomotive 100 slows. This electronic control capability mayprovide a smooth, linearly extended range operation of dynamic brakingto a lower locomotive speed than would be obtainable through traditionalsystems that simply switch between different fixed resistance values.

Armature choppers 212 may be separately controllable, so that theequivalent resistance of each grid resistor 211 can be individuallychanged. In one embodiment, this may be used for wheel slip correction.This may provide opportunities for multivariable control for low speed,part-brake handle operation, in addition to wheel slipping conditions.The armature choppers 212 may be used to reduce the braking effort onone or more axles by reducing the armature chopper 211 current, therebyincreasing the equivalent resistance of a single path and reducing thearmature 104 current, and correcting the slip.

FIG. 3 provides a flowchart 300 illustrating a method of controllingarmature chopper 212 to decrease the effective resistance of gridresistor 211. Armature chopper 212 may be a switched DC currentregulation device arranged in parallel with grid resistor 211 and inseries with armature 104. Armature chopper 212 may be arranged in serieswith armature 104 and in parallel with grid resistor 211.

The method may include monitoring a field current associated with fieldwinding 105 (Step 302). Field winding 105 may be arranged in parallelwith the series combination of grid resistor 211 and armature 104. Fordynamic braking, as locomotive 100 slows down, field chopper 208 mayincrease the field current associated with and travelling through fieldwinding 105 so that armature 104 can continue to generate current. Inone embodiment, field winding subsystem 202 may include a current sensorpositioned on one or more of the field windings and may be configured tomonitor the field current passing through there. In another embodiment,other types of electrical sensors may be used, such as a voltage sensor,to calculate the field current.

The method may also include comparing the field current with a thresholdvalue (Step 304). In one embodiment, the threshold value may relate tothe current rating of field windings 105. In another embodiment, thethreshold value of the field current may depend on the characteristicsof field chopper 208. If the field current has not reached the thresholdvalue (Step 306: No), the method may revert back to step 302 andcontinue monitoring field current.

As locomotive 100 continues to slow and the field current through fieldwinding 105 reaches a threshold value, increasing the field currentabove the current rating of the field windings is not a viable optionfor ensuring linear and continuously-variable control of dynamic brakingat low machine speeds. Thus, when current through the field windingreaches the threshold value (Step 306: Yes), traction motor drive system200 may selectively operate armature chopper 212 to decrease theequivalent resistance of grid resistor 211 for continued control ofdynamic braking at low speed.

If the field current has reached a threshold value (Step 306: Yes), themethod may include calculating a duty cycle associated with armaturechopper 212 (Step 308). The effective resistance of grid resistor 211can be decreased by increasing the duty cycle of armature chopper 212.According to one embodiment, the duty cycle of armature chopper 212 maybe inversely related to the current generated by armature chopper 212and to the effective resistance of grid resistor 211.

It is contemplated that the duty cycle of the chopper may be variedbased on a variety of different parameters. In one embodiment, the dutycycle may change as a function of the current generated by armature 104.That is, armature chopper 212 may be configured to conduct current basedon the rotational velocity of armature 104. The duty cycle may alsodepend on the current measured at armature chopper 212. The duty cyclemay also be a function of the rotational velocity of armature 104. Inone embodiment, armature chopper 212 may be configured to conductcurrent based on the rotational velocity of armature 104. To graduallydecrease the braking effects of armature 104, the calculated duty cycleof armature chopper 212 will gradually be increased. In one embodiment,the duty cycle may linearly increase as a function of time.

Other methods of controlling dynamic braking are known by those havingordinary skill in the art. For example, dynamic braking controls maydepend on the grid current, in which the algorithm associates theoperator's brake handle position with the current flow through gridresistor 211. In another embodiment, dynamic braking controls may dependon the field current, in which the brake handle regulates the currentthrough field windings 105. In yet another embodiment, dynamic brakingcontrols may depend on the brake effort, in which the brake effort, ordynamic braking power, is related to the brake handle position.

The method may also include operating armature chopper 212 at thecalculated duty cycle to selectively bypass grid resistor 211 during theoperating portion of the calculated duty cycle when the field currentexceeds the threshold value (Step 310). Armature chopper 212 may be aswitched DC current regulation device that may be operated at a variableduty cycle. When armature chopper 212 is “on” grid resistor 211 iseffectively shorted out of the circuit causing the current to bypassgrid resistor 211. When armature chopper 212 is “off,” no current isallowed to bypass grid resistor 211, causing any current that armature104 generates to travel through grid resistor 211. In one embodiment,the method may also include operating armature chopper 212 at 0% dutycycle—that is, operating as an open circuit—until the field currentreaches the threshold value.

According to one exemplary embodiment, the presently disclosed systemsand methods for continuously varying the dynamic braking capability atlow locomotive speeds may be implemented with the ability to isolatetraction motor components. As such, should one or more grid resistors211 fail during operation of traction motor drive system 200, thecorresponding armature subsystem 201 may be isolated from the system.The remaining armature subsystems 201 can then be operated to compensatefor the loss of braking capacity associated with the isolated armaturesubsystem 201. Thus, by providing a method for electronicallycontrolling the effective resistance of the grid resistors 211 bymodifying the duty cycle of the armature choppers 212, the systems andmethods described herein can be easily modified in real-time to maintainthe dynamic braking control capabilities of traction motor drive system200, in real-time, as locomotive 100 conditions require.

FIG. 4 shows one application of dynamic braking with a traction motordrive system capable of isolating both armature subsystems 201 and fieldwindings 105. Traction motor drive system 200 may be implemented withone of the isolation capabilities. For example, traction motor drivesystem 200 may include field winding isolation capabilities withoutnecessarily requiring an armature isolation system. Alternatively,traction motor drive system 200 may be provided with armature isolationcapabilities and without the field isolation capabilities. Thus, thesystem need not be limited to the specific embodiment of FIG. 4 but mayhave different configurations of the components described. Each type ofisolation capability is discussed in turn.

Traction motor drive system 200 may be capable of isolating at least onearmature subsystem 201 in which one or more components ismalfunctioning. To isolate armature subsystem 201, the subsystem may beelectrically disconnected from the remainder of traction motor drivesystem 200. Armature isolation may be realized by the selectiveoperation of a power switch 401 and motor-brake switch 209 connected toarmature 104.

For traction motor isolation, motor-brake switch 209 may be asingle-pole, triple-throw switch that is able to isolate armature 104from the remainder of traction motor drive system 200 in the event of anelectrical failure affecting all or part of armature subsystem 201.According to an exemplary embodiment of FIG. 4, motor-brake switch 209may have at least three modes of operation: a powering mode, a brakingmode, and an isolation mode. During the powering mode, motor-brakeswitch 209 connects the second terminal of armature 104 to negativevoltage source 204. In the second position, motor-brake switch 209 mayelectrically couple the second armature terminal to positive voltagesource 203 to shift armature 104 into braking mode. To isolate armaturesubsystem 201, motor-brake switch 209 shifts into a third mode thatelectrically disconnects the second terminal of armature 104 from anypower source. This, in cooperation with the operation of power switch401, electrically isolates armature 104 from the remainder of tractionmotor drive system 200.

In one embodiment, motor-brake switch 209 may be configured to isolatearmature 104 automatically in the event of an electrical failureaffecting all or part of armature subsystem 201. In another embodiment,motor-brake switch 209 may be configured to isolate armature 104 onlyafter receiving a command from an operator or another system oflocomotive 100 to isolate armature 104. There are a variety of otherswitches and contactors known in the art that are capable ofdisconnecting armature 104 that are equally suitable to operate asmotor-brake switch 209 of the traction motor drive system 200.Motor-brake switch 209 may include or embody any of these types ofcomponents.

Power switch 401 may be a single-pole, single-throw switch that is ableto isolate armature subsystem 201 from the remainder of the tractionmotor drive system 200 by disconnecting the armature subsystem 201 frompositive voltage source 203. In one embodiment, power switch 401 mayoperate to isolate armature 104 automatically in the event of anelectrical failure affecting all or part of armature subsystem 201.Alternatively, power switch 401 could operate to isolate armature 104only after receiving a command from an operator or another system of thelocomotive 100 to isolate armature 104. There are a variety of otherswitches and contactors known in the art that are capable ofdisconnecting armature 104 that are equally suitable for operating aspower switch 401 of traction motor drive system 200. Power switch 401may include or embody any of these types of components.

In the exemplary circuit of FIG. 4, motor-brake switch 209 and powerswitch 401 are configured to isolate armature subsystem 201, includingarmature 104, grid resistor 211, and armature chopper 212, from theremainder of traction motor drive system 200. In another embodiment,motor-brake switch 209 could be a single-pole, double-throw switchcapable only of switching between braking mode and powering mode. Inthis configuration, to achieve armature isolation, a dedicated isolationswitch (not shown) could be incorporated to achieve the same result.

While not shown in FIG. 4, alternative configurations of traction motordrive system 200 may include fewer power switches 401 and motor-brakeswitches 209, such that each power switch 401 and motor-brake switch 209controls the current flow to multiple armature subsystems 201. It is notnecessary that each armature subsystem 201 have a devoted power switch401 and motor-brake switch 209. For example, pairs of armaturesubsystems 201 could share a common power switch 401 and a commonmotor-brake switch 209. Other configurations of armature isolationcomponents can be contemplated by one with ordinary skill in the art.

In addition to armature isolation, traction motor drive system 200 maybe configured to isolate defective or malfunctioning field windings 105using a field isolation system 402 associated with field windingsubsystem 202. Field isolation system 402 comprises a shunt circuit 403,a first field switch 404, and a second field switch 405. Traction motordrive system 200 may include a plurality of field isolation systems 402,each field isolation system 402 associated with a respective one of thefield windings 105. Alternatively, each field isolation system 402 maybe associated with a plurality of field windings 105. In FIG. 4,traction motor drive system 200 contains three field isolation systems402, each corresponding with a pair of field windings 105.

Within field winding subsystem 202, field windings 105 are connected inseries with first field switch 404 and second field switch 405, whichcan remove defective field winding 105 from traction motor drive system200. By shunting defective field winding 105, the remaining fieldwindings 105 of traction motor drive system 200 continue to receivepower and operate normally. The embodiment illustrated in FIG. 4 allowstraction motor drive system 200 to achieve ⅔ of normal tractive orbraking effort despite a malfunctioning field winding 105. To isolate apair of field windings 105, first field switch 404 connects to the firstend of shunt circuit 403, and second field switch 405 connects to thesecond end of shunt circuit 403. In this configuration, field windings105 are shunted, such that the current continues to flow through theremainder of field winding subsystem 202. In another embodiment, a fieldwinding subsystem 202 could equally be applied to each of the sixrepresentative field windings 105, providing the ability to isolate anysingle defective circuit, while preserving the functionality of theremaining 5, providing 5/6 of the normal electrical braking effort.

Field switches 404, 405 can be any electromechanical component capableof isolating field winding 105 from the remainder of traction motordrive system 200 in the event of an electrical failure affecting all orpart of field winding 105. In one embodiment, field switches 404, 405may be single-pole, double-throw switches. There are a variety of otherswitches and contactors known in the art that are capable of isolatingfield winding 105 from the remainder of traction motor drive system 200.Field switches 404, 405 may include or embody any of these types ofcomponents.

The operation of field switches 404, 405 may be automatic or manual. Inone embodiment, field switches 404, 405 could operate to shunt one ormore of the field windings 105 automatically in the event of anelectrical failure affecting all or part of a malfunctioning fieldwinding 105. Alternatively, field switches 404, 405 could operate toshunt field winding 105 only after receiving a command from an operatorto isolate field winding 105 from the remainder of traction motor drivesystem 200. In yet another embodiment, the operation of field switches404, 405 could result from a combination of automatic or manual inputs.For example, first field switch 404 may operate to shunt field winding105 only after receiving a command to do so, and second field switch 405may operate automatically once first field switch 404 becomes engaged.

It should be emphasized that power switch 401 and motor-brake switch209, as well as first and second field switches 404, 405, can beseparately controlled such that isolation of armature subsystem 201 doesnot require isolation of field winding 105. Likewise, isolation of fieldwinding 105 does not require isolation of armature subsystem 201.

It is contemplated that locomotive 100 may include additional componentsfor communication between an operator and traction motor drive system200, such as a controller. In one embodiment, the controller may be aprocessor capable of receiving inputs from sensors to detect electricalfailures. The controller may also be configured to notify the operatorof the occurrence of an electrical fault and may allow the operator tosend control signals to isolate the affected components. Locomotive 100may include an operator interface that provides the operator a way toreceive fault notifications and send commands to the controller. Forexample, the operator interface may include a processor for receivingnotifications from the controller and an output screen for displayingthese notifications to the operator. The operator interface may alsoinclude an operator input system, such as a series of buttons, for theoperator to send commands to the controller to selectively isolateelectrical components.

Additionally or alternatively, locomotive 100 may include a brakingcontroller. This braking controller may monitor the operability ofdynamic braking and a second form of braking, such as pneumatic braking.The braking controller may be able to turn on and off the differentbraking systems of locomotive 100.

Once the conduction current of armature choppers 212 has reached itsmaximum, additional braking systems may be necessary. To account forthis, the controller may send a communication signal to a brakingcontroller to engage a secondary braking system, such as a pneumaticbraking system.

INDUSTRIAL APPLICABILITY

The disclosed systems and methods for dynamic braking described hereinprovide a robust solution for enhancing the performance of tractionmotor drive systems by allowing smooth, linear, extended-range operationof dynamic braking. By enabling dynamic braking at lower locomotivespeeds, the systems and methods described herein further reduce thereliance on friction-based braking components at these low locomotivespeeds. As a result, service costs associated with repair andreplacement of wear components (e.g., brake pads, brake rotors) may besignificantly reduced. Furthermore, damage associated with wheel heating(due to braking friction) may be dramatically reduced.

The presently disclosed traction motor drive system may have severaladvantages. Specifically, the presently disclosed dynamic braking systemoperates at much lower locomotive speeds than traditional dynamicbraking systems. In blending braking systems, the more a locomotive canrely on dynamic brakes rather than air brakes, the less frequently themechanical braking parts require maintenance and replacement. Extendingthe time between scheduled maintenance maximizes the amount of timelocomotives are in service.

Additionally, the traction motor drive system replaces conventionalstepped resistors with a switched DC current regulation device, so thatthe dynamic braking system applies braking force to the locomotivelinearly, rather than in a step-down method of conventional dynamicbraking systems. As a result, locomotive handling becomes easier becausethe locomotive does not experience the lunging associated with step-downdynamic braking.

Furthermore, the disclosed dynamic braking solution works for tractionmotor drive systems in which the armature and field windings areconnected in parallel. This increases the stability of the tractionmotor drive system and allows electrical isolation of grid resistorswhen not in use.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed systems andassociated. Other embodiments of the present disclosure will be apparentto those skilled in the art from consideration of the specification andpractice of the present disclosure. It is intended that thespecification and examples be considered as exemplary only, with a truescope of the present disclosure being indicated by the following claimsand their equivalents.

1-8. (canceled)
 9. A method of controlling an armature chopper arrangedin series with an armature and in parallel with a grid resistor todecrease the effective resistance of the grid resistor when the armatureis generating current, the method comprising: monitoring a field currentassociated with a field winding, the field winding arranged in parallelwith the grid resistor and the armature; comparing the field currentwith a threshold value; and when the field current exceeds a thresholdvalue: calculating a duty cycle associated with the armature chopper;and switching the armature chopper at the calculated duty cycle toselectively bypass the grid resistor during the portion of a period thearmature chopper is on.
 10. The method of claim 9, further includingoperating the armature chopper at 0% duty cycle until the field currentreaches the threshold value.
 11. The method of claim 9, wherein the dutycycle is based on the current generated by the armature.
 12. The methodof claim 9, further including linearly increasing the duty cycle of thearmature chopper.
 13. The method of claim 12, further includingincreasing the duty cycle of the armature chopper to 100% to completelybypass the grid resistor. 14-20. (canceled)